Formation,magnetic And Magnetocaloric Properties Of The RE-TM(RE=Gd,Dy;TM=Co,Ni) Binary Amorphous Alloys

Based on the magnetocaloric effect(MCE)of materials,a new refrigeration technology,namely the magnetic refrigeration(MR),has been developed in recent years.Compared to the traditional vapor compression/expansion refrigeration technology,the MR technology shows broad application prospect because of its advantages such as high efficiency,energy saving,compactness and environmentally friendliness.As the key issue of the MR technology,the development of MCE materials has received extensive attentions,and thus numerous MCE materials have been developed in the last few decades.Amongst these,the amorphous MCE materials,especially the rare-earth(RE)based metallic glass,are considered to be suitable candidates for constructing the table-like magnetic entropy change(-?S_m)curve necessary for the Ericsson cycle because of their excellent magnetocaloric properties and their tunable Curie temperature(T_c).In addition,compared to the materials that undergo a first-order magnetic phase transition(FOMPT),the amorphous alloys show better mechanical properties and corrosion resistance,low eddy current loss due to their high electronic resistance.However,there are still some questions,such as the compositional range of the metallic glasses,the compositional dependence of the Curie temperature and the magnetocaloric properties,and mechanism for the origin of coercivity in some of the RE-based metallic glasses and its effect on magnetocaloric properties,being not yet clearly described.Therefore,we selected the binary RE-TM metallic glasses as the simple and ideal model for the systematical investigation of the above questions.Finally,the RE-TM-based amorphous composites with table-like magnetic entropy change curve from temperature lower than the ice point of water to the temperature higher than the room temperature will be constructed.The results and conclusions obtained in this thesis are as follows:(1).First of all,RE-TM(RE=Gd,Dy;TM=Co,Ni)ribbons with an average thickness of?40?m were prepared.The amorphous characteristics of these ribbons were checked by the X-ray diffraction patterns.The glass transition temperature and crystallization temperature of the amorphous ribbons were obtained from the differential scanning calorimeter.The glass formability(GFA)of the amorphous ribbons was studied.It was found that the best glass former in the Gd-Ni and Dy-Co binary alloy systems is the alloys with eutectic composition,while in the Gd-Co and Dy-Ni binary alloy systems is the ones with composition slightly deviated from the eutectic point.The slightly deviation from the eutectic point of the composition for the best glass former in the Gd-Co binary alloys is possibly because that its eutectic point is too close to the Gd₁₂Co₇ intermetallic compound,while in Dy-Ni binary alloys is most likely due to the pseudo-eutectic solidification of the melt.(2).The magnetic properties and magnetization behaviors of the RE-TM(RE=Gd,Dy;TM=Co,Ni)binary amorphous alloys were investigated systematically.It was found that the compositional dependence of T_c in the Gd-TM binary amorphous alloys obeys a linear relationship,while in the Dy-Co binary amorphous alloys is not linear.The non-linear relationship between the T_c and the composition of the Dy-Co binary amorphous alloys is most likely due to the non-linear compositional dependence of 3d-4f interaction,which is negligible or does not exist in the Gd-TM binary amorphous alloys.The 3d-3d interaction and 4f-4f interaction,both of which exist in either the Gd-TM or the Dy-TM metallic glasses,vary linearly with the composition of the binary alloys.On the other hand,in contrast to the isotropic 3d-3d interaction and 4f-4f interaction,the 3d-4f interaction is anisotropic.The randomly frozen 3d-4f interaction at the temperature well below the freezing temperature(T_f)may lead to the spin-glass-like behaviors and the high coercivity in the Dy-TM amorphous alloys.(3).The magnetocaloric properties of the RE-TM binary amorphous alloys were studied.The Gd-TM binary amorphous alloys show typical magnetocaloric behaviors of the second order magnetic phase transition(SOMPT)materials.The spin-glass-like behavior and the coercivity in the Dy-TM metallic glasses obviously deteriorate their magnetocaloric properties at low temperature,and even make the magnetocaloric behaviors of these amorphous alloys irreversible.Most important of all,a binary Gd₅₀Co₅₀ amorphous alloy was found to exhibit good MCE near the ice point of water,which make it possible to develop amorphous alloys with better MCE at temperature above the ice point of water or even the room temperature.(4).Based on the above results,Gd-Co-based MCE materials with T_c near room temperature have been developed.By improving the cooling rate,we successfully enhanced the Curie temperature of Gd-Co binary amorphous alloys to above the ice point of water in a Gd₄₈Co₅₂ amorphous ribbon.The glass formability and Curie temperature of the Gd₅₀Co₅₀ amorphous alloy were further improved by minor Ni and/or Fe substitution for Gd,and thus several pseudo-binary Gd-TM amorphous alloys with Curie temperature ranging from 267 K to 310 K were developed.Although the peak value of-?S_m(-?S_m^peak)in the Gd-Co-based amorphous alloys decrease with the improving T_c,their peak values of adiabatic temperature rise(?T_ad)are still much higher than those of Fe-Zr-B-based amorphous alloys.Finally,several Gd-Co-based amorphous composites with table-like-?S_m curve were constructed.The average-?S_m platform values of the Gd-Co-based amorphous composites were much higher than that of the Fe-Zr-B-based amorphous composite.Combined with the much higher?T_ad,the much higher COP is expected to be achieved in the Gd-Co-based amorphous composites.